1. Field of the Invention
The present invention relates to a balancing structure for a motor. In particular, the present invention relates to a balancing structure for maintaining stable rotation of a rotor of a motor.
2. Description of Related Art
A wide variety of structures for maintaining rotational balance for a rotor of a motor have been proposed. One of these structures comprises a fixed portion, a rotor, an attracting portion, and a magnetically conductive portion. The fixed portion includes a base, an axial tube fixed on the base, a bearing received in the axial tube, a stator, and a circuit board. The stator includes a coil with axial winding or radial winding and a plurality of pole plates (or pole arms). The rotor includes a shaft rotatably extending through the bearing and an annular magnet surrounding the pole faces of the pole plates. The attracting portion is provided on a bottom of the rotor or a top end of the axial tube. Alternatively, the attracting portion is provided by the annular magnet or the alternating magnetic fields created by the pole plates of the stator. The magnetically conductive portion is made of a magnetically conductive material and may be comprised of a disc with two arcuate edges, a casing fixed in a rotor housing of the rotor, an annular plate, a plurality of arcuate plates, or a rotor housing of a spindle motor. The magnetically conductive portion may be provided on the circuit board, an inner periphery of the rotor, or the bottom of the rotor, and associated with the attracting portion. Such a structure is disclosed in, e.g., Taiwan Utility Model Publication Nos. 383818, 422365, 428838 and M241969, U.S. Pat. Nos. 6,097,120; 6,483,209; 6,700,241; and 6,727,626, and U.S. Patent Publication No. 2005/0006962.
When the motor turns, alternating magnetic fields are created by the pole faces of the magnetic pole plates (or pole arms). The magnetically conductive portion is attracted by the attracting portion during rotation of the rotor, thereby maintaining rotational balance of the rotor, avoiding disengagement of the rotor shaft from the stator, reducing rotational friction of the bearing, and prolonging the life of the motor.
It is common to make the size of the metal plate consisting of the magnetically conductive portion as large as possible so as to provide a large area facing the magnet of the attracting portion (such as the annular magnet). This may result in excessive attracting force between the magnetically conductive portion and the attracting portion for balancing the rotor. The fixed portion generally includes a circuit board with at least one Hall element that faces the annular magnet of the rotor for detecting a change in polarity of the annular magnet when the rotor turns. The Hall element is generally fixed on the circuit board by a number of legs that are bent. The Hall element inevitably warps upward and thus could not lie flat on the circuit board. The assembling precision of the Hall element is thus adversely affected. Further, it becomes more and more difficult to mount additional positioning elements on the fixed portion for precisely positioning the Hall element due to the limited assembling space provided by the fixed portion on the trend of miniaturization. The quality of the resultant motors after assembly is not uniform. More specifically, the Hall elements in some of the motors could not provide accurate detection signal. The rotational efficiency of the rotor is adversely affected and the qualified product ratio is lowered.